Tag Archives: study

(Physorg; University of Exeter; 10 May; Photo: Alex Thornton)

Jackdaws recognise each other’s voices and respond in greater numbers to warnings from familiar birds than strangers, new research shows.

The birds produce a harsh “scolding call” when they spot a predator, calling fellow jackdaws to mob the intruder and drive it away.

University of Exeter researchers have discovered that each bird has a unique call, and the size of the mob depends on which bird calls the warning.

The scientists played recordings of individual calls and found that the largest mobs assembled when birds heard the cry of a member of their own colony.

“Joining a mobbing event can be dangerous, as it involves approaching a predator, so it makes sense for individuals to be selective in whom they join. Our results show that jackdaws use the ability to discriminate between each other’s voices when deciding whether to join in potentially risky collective activities,” said Dr Alex Thornton, of the Centre for Ecology and Conservation on the University of Exeter’s Penryn Campus in Cornwall.

“We also found a positive feedback loop – if birds joining a mob made alarm calls of their own, this in turn caused more birds to join in, magnifying the size of the mob.”

The researchers studied wild jackdaws, a highly social member of the crow family, as part of the Cornish Jackdaw Project, a long-term study of jackdaw behaviour and cognition in sites across Cornwall.

In playbacks at nest-box colonies during the breeding season, they broadcast the warning calls of a resident from each nest-box, another member of the same colony, a member of a different colony, and a rook (a species that often associates with jackdaws).

Jackdaws were most likely to respond to a warning from a bird from the resident nest-box owner, followed in turn by other colony members, non-colony members and rooks.

Responses were also influenced by caller sex, with jackdaws less likely to echo a warning if the caller was a female stranger from a different colony.

The paper, published in the journal Scientific Reports, is entitled: “Caller characteristics influence recruitment to collective anti-predator events in jackdaws.”

With the arrival of spring, we look forward to the return of hundreds of species of migratory songbirds from their wintering grounds.

Sparrows, swallows, warblers and thrushes, among other songbirds, will be returning from their wintering sites anywhere between the southern United States and distant South America.

Some of these birds will return with a small “backpack” that has recorded their entire migration from their North American breeding grounds to their wintering grounds and back.

Birds provide important ecosystem services, such as preying on insects, dispersing seeds, scavenging carcasses and pollinating plants. Unfortunately, there have been dramatic declines in many migratory songbirds over the past few decades, with some of these populations dropping by more than 80 per cent.

If we are to find ways to slow or reverse these declines, we must first figure out what’s causing them. Climate change, habitat loss and predation by cats are among the leading causes of bird declines.

But with the vast distances these birds move over the course of the year, it can be difficult to pinpoint the main cause for a given species —and where it’s occurring.

Migratory connections

To answer this question, we need to know where individual birds spend their time throughout the year.

We have a good idea of the range —or the total area —the birds occupy during the breeding and wintering periods. But ranges are composed of many populations, and we still have a very poor understanding of how individuals within each of these populations are connected between seasons.

Individuals from different breeding populations may remain segregated during the winter. For example, some ovenbirds winter in the Caribbean whereas others spend their winters in Mexico and Central America.

Or a bird may mix with individuals that originate from other breeding populations, such as bobolinks that mix in South America during the winter.

These patterns of migratory connectivity have critical implications for predicting how migratory songbirds will respond to environmental change.

Habitat loss —deforestation, for example —in one place can have different effects. If habitat loss occurs in a wintering area where breeding populations mix, it may have wide-ranging, yet diffuse, effects on the breeding populations. But if the habitat loss occurs in a wintering area that is occupied by a single breeding population, the effect may be more focused.

For example, habitat loss in South America will likely have range-wide effects on bobolinks, while habitat loss in the Caribbean may only influence a portion of the breeding populations of ovenbirds.

Backpacks for birds

We know that the breeding and wintering populations of most species mix to some extent, but…

(Mariëtte Le Roux; Nature Ecology & Evolution; 7 May 2018)

Why have some birds opted for a taxing life of constant migration—seeking out temperate climes to feed as winter arrives, only to return months later to breed?

Seemingly paradoxically, the behaviour is driven by a quest for energy efficiency, a study said Monday.

Migrating birds, researchers found, gain more energy from whatever is on the destination menu than they expend getting there and back, or could find without making the trek.

Why don’t they just stay in the warm place? Because there is too much competition for food with other species, said the study published in the journal Nature Ecology & Evolution.

Instead, they return to their cold, northern hemisphere home where they don’t have to fight others for the food there is.

The work “provides strong support for the hypothesis that birds distribute themselves in an optimal way in terms of energy,” study co-author Marius Somveille of the University of Oxford’s zoology department told AFP.

While it was known that birds migrate in search of food, it has remained a puzzle why they have adopted this exacting lifestyle.

The new study explains the behaviour of not only migratory birds, but also that of sedentary or “resident” ones, its authors said.

These too weighed the available food against greener pastures, and came to a different conclusion.

Most resident birds are found in the tropics, where food is easier to get by.

Fly or die

The study used a theoretical model to examine why birds migrate—about 15 percent of the total—while others do not.

It started with a model world with similar climatic differences between regions than our real one.

The researchers then added virtual birds, and the estimated amount of “energy”, or food, available in different regions.

Given these inputs, the model birds dispersed very similarly to what happened in real life.

The birds started off in the food-rich tropics, but growing competition forced some to start moving further afield.

“In our increasingly crowded virtual world, species progressively started exploiting more extreme pockets of seasonally available energy supply, often migrating longer distances,” the team wrote.

The model adds to our understanding of how Earth’s plants and animals came to be distributed as they are, the researchers added.

It could also be useful in predicting the future movements of other animals—to determine how they might migrate in response to global warming, for example.

In spring songbirds greet the rising and setting sun with a cacophony
of chirps meant to entice mates and claim territory. But artificial light
has made the night sky brighter and disrupted the seasonal rhythms
of birds that use day length as a cue to sing. Of six songbird species
that scientists studied in Germany, four started singing earlier in the year
because of night lighting. The long-term effects of light pollution on
birds’ ecosystems, and their survival, remain unclear.

(University of Cambridge 18 Dev 2017)

Many animals have evolved to stand out. Bright colours are easy to spot, but they warn predators off by signalling toxicity or foul taste.

Yet if every individual predator has to eat colourful prey to learn this unappetising lesson, it’s a puzzle how conspicuous colours had the chance to evolve as a defensive strategy.

Now, a new study using the great tit species as a “model predator” has shown that if one bird observes another being repulsed by a new type of prey, then both birds learn the lesson to stay away.

By filming a great tit having a terrible dining experience with conspicuous prey, then showing it on a television to other tits before tracking their meal selection, researchers found that birds acquired a better idea of which prey to avoid: those that stand out.

The team behind the study, published in the journal Nature Ecology & Evolution, say the ability of great tits to learn bad food choices through observing others is an example of “social transmission.”

The scientists scaled up data from their experiments through mathematical modelling to reveal a tipping point: where social transmission has occurred sufficiently in a predator species for its potential prey to stand a better chance with bright colours over camouflage.

“Our study demonstrates that the social behaviour of predators needs to be considered to understand the evolution of their prey,” said lead author Dr Rose Thorogood, from the University of Cambridge’s Department of Zoology.

“Without social transmission taking place in predator species such as great tits, it becomes extremely difficult for conspicuously coloured prey to outlast and outcompete alternative prey, even if they are distasteful or toxic.

“There is mounting evidence that learning by observing others occurs throughout the animal kingdom. Species ranging from fruit flies to trout can learn about food using social transmission.

“We suspect our findings apply over a wide range of predators and prey. Social information may have evolutionary consequences right across ecological communities.”

Thorogood (also based at the Helsinki Institute of Life Science) and colleagues from the University of Jyväskylä and University of Zurich captured wild great tits in the Finnish winter. At Konnevesi Research Station, they trained the birds to open white paper packages with pieces of almond inside as artificial prey.

The birds were given access to aviaries covered in white paper dotted with small black crosses. These crosses were also marked on some of the paper packages: the camouflaged prey.

One bird was filmed unwrapping a package stamped with a square instead of a cross: the conspicuous prey. As such, its contents were unpalatable — an almond soaked with bitter-tasting fluid.

The bird’s reaction was played on a TV in front of some great tits but not others (a control group). When foraging in the cross-covered aviaries containing both cross and square packages, the birds exposed to the video were quicker to select their first item, and 32% less likely to choose the ‘conspicuous’ square prey.

“Just as we might learn to avoid certain foods by seeing a facial expression of disgust, observing another individual headshake and wipe its beak encouraged the great tits to avoid that type of prey,” said Thorogood.

“By modelling the social spread of information from our experimental data, we worked out that predator avoidance of more vividly conspicuous species would become enough for them to survive, spread, and evolve.”

Great tits — a close relation of North America’s chickadee — make a good study species as they are “generalist insectivores” that forage in flocks, and are known to spread other forms of information through observation.

Famously, species of tit learned how to pierce milk bottle lids and siphon the cream during the middle of last century — a phenomenon that spread rapidly through flocks across the UK.

Something great tits don’t eat, however, is a seven-spotted ladybird. “One of the most common ladybird species is bright red, and goes untouched by great tits. Other insects that are camouflaged, such as the brown larch ladybird or green winter moth caterpillar, are fed on by great tits and their young,” said Thorogood.

“The seven-spotted ladybird is so easy to see that if every predator had to eat one before they discovered its foul taste, it would have struggled to survive and reproduce.

“We think it may be the social information of their unpalatable nature spreading through predator species such as great tits that makes the paradox of conspicuous insects such as seven-spotted ladybirds possible.”

(Oregon State University 15 Dec 2017;Photo:Hankyu Kim)

Old forests that contain large trees and a diversity of tree sizes and species may offer refuge to some types of birds facing threats in a warming climate, scientists have found.

In a paper published in Diversity and Distributions, a professional journal, researchers in the College of Forestry at Oregon State University reported that the more sensitive a bird species is to rising temperatures during the breeding season, the more likely it is to be affected by being near old-growth forest.

Researchers studied 13 bird species that have been tracked annually in the U.S. Geological Survey’s annual Breeding Bird Survey, one of the most comprehensive efforts of its kind in North America. Only two — the Wilson’s warbler and hermit warbler — showed negative effects from rising temperatures over the past 30 years, but actual counts of both species show that their populations are stable or increasing in areas that contain high proportions of old-growth forest.

A team led by Matthew Betts, professor in the College of Forestry, reached their conclusions by analyzing data for bird populations, forest structure and climate across northwestern North America. The researchers used satellite imagery to determine the amount of old-growth forest within about 450 yards of each 25-mile-long bird survey route.

The findings provide an additional reason for old-growth forest conservation, said Betts. “Managers hoping to combat the effects of climate change on species’ populations may now have an additional tool — maintaining and restoring old-growth forest.” He noted that this is important because management recommendations from biodiversity and climate studies have traditionally been sparse. Such studies have tended to focus on moving species to cooler climates or simply reducing carbon emissions.

Wilson’s warbler winters in Mexico and breeds during the late spring and early summer along the West Coast and across northern North America from Alaska to New England and the Canadian Maritimes. Although it occurs in early-stage as well as mature forests, it is declining at a rate of about 2 percent per year in the Pacific Northwest.

The hermit warbler also winters in Mexico but breeds exclusively along the West Coast as far north as Washington. Its populations are relatively stable but declining in landscapes with low amounts of old-growth forest.

Additional research will be needed to identify the specific features of mature forests that buffer the effects of warming temperatures on birds. One possibility, the researchers said, is that the large trees themselves function as “heat sinks” during warm periods and thus moderate temperatures. Multiple canopy layers may also provide climate buffering effects.

(Kyle Elliott, Mehrnoosh Azodi, 23 November 2017;Photo: CC BY)

For 47 years, biologists have plucked eggs from seabird nests along the British Columbia coast. Many of the eggs were collected from remote rocky islands surrounded by some of the world’s roughest seas.

In all, they collected 537 eggs from six species, including ancient murrelets, rhinoceros auklets and double-crested cormorants. Now these eggs are revealing new information about the way mercury finds its way into the ecosystem.

The eggs of top predators, like these seabirds, are important for researchers who study chemical pollutants. We use these eggs to understand, for example, how mercury levels in seabirds have changed over time. This data helps us understand whether control measures, including international agreements, are reducing mercury levels in the global environment.

Human activities, especially the burning of coal, release mercury into the atmosphere. Mercury levels in the Pacific Ocean are now three to five times higher than they were before the industrial revolution. Our activities have released 1.5 million tonnes of mercury into the air, land and water bodies since 1850.

For wildlife —and humans — mercury is a neurotoxin. High levels in the food chain can have negative impacts on the reproductive health of seabirds, marine mammals and other carnivores that feed on seafood. For example, seabirds with heavy mercury loads are less likely to breed. Mercury also slows down the healthy growth of chicks.

Bacteria plays a fundamental role

Researchers had thought that the seabirds that feed on large predatory fish would have the highest levels of mercury because large fish, which are higher up the food chain, accumulate more mercury in their bodies than the small fish and invertebrates that occupy the bottom of the food chain.

But later studies contradicted these expectations. By studying mercury levels in the eggs of seabirds over many years, researchers found seabirds that fed on invertebrates and small fish had higher mercury levels than the seabirds that ate large fish.

Our research explained why the highest mercury levels occurred in species that ate small fish. We measured mercury levels and a collection of dietary tracers called stable isotopes in the seabird eggs. We studied nitrogen, carbon and sulphur isotopes because these chemicals tell us about the type of food the birds were eating.

We found that the seabird eggs with high mercury levels also had higher sulphate levels. It meant that these birds were eating small fish from areas that were also favoured by a specific type of bacteria.

These bacteria, called sulphate-reducing bacteria, convert mercury to methylmercury, the toxic form of mercury that can move up the food web and harm seabirds. These bacteria tend to be found in deep-water sediments.

Stable mercury levels

The study also showed that mercury concentrations in Pacific seabird eggs have been relatively stable over the past 50 years. This is surprising, given the dramatic changes in mercury concentrations in Pacific waters.

A decline in surface-dwelling fish stocks in the Pacific Ocean has forced some seabirds to feed elsewhere where there are fewer sulphate-reducing bacteria. By changing their diet, these seabirds end up with lower mercury levels.

Monitoring programs have suggested that environmental levels of mercury are in decline, but this research suggests otherwise. Based on our results, we believe monitoring programs need to consider the structure of the food web, including where these birds are foraging.

Human health threat

Although this research focuses on the eggs of seabirds, it has implications for human health. The World Health Organisation has identified mercury as one of the top ten chemicals of major health concern.

Mercury pollution is a global issue since elemental mercury travels over long distance and methylmercury accumulates in the food chain, posing a serious risk to both humans and wildlife who feed on seafood.

The global scale of this issue was the key driving force for a global environmental treaty called the Minamata Convention, which enforces reductions on mercury emissions. The agreement entered into force on Aug. 16, 2017, making it the most recent international law that aims to protect humans and the environment from the threat of toxic mercury.

The current study on avian predators, which feed on marine food webs similar to those we feed on ourselves, illustrates how important it is to consider those food webs when considering the risk of mercury.

Rather than simply considering larger species high in the food chain as being riskiest, we suggest that governments also consider the role of bacteria and reduce the consumption of those fish feeding in food webs with high levels of sulphate-reducing bacteria.